U.S. patent application Ser. No. 10/051,122 (“Bell”) discloses a conventional robust heterodyne interferometer which incorporates a planar lightwave circuit (“PLC”), for precisely measuring distances. The conventional interferometer typically propagates a measurement optical signal through a single mode optical fiber or waveguide, where the optical signal exits the optical fiber or waveguide via an exit aperture into free space before passing through a collimating lens. The measurement optical signal is reflected off at least one reflective surface, and is returned through a collimating lens, leaving free space via the entrance aperture of an optical fiber or waveguide. The measurement beam is then processed in optical solid space according to known techniques, to precisely measure the distance or change of distance of the measurement beam path. According to at least one conventional configuration, the measurement beam passes in and out of the PLC via one optical fiber or waveguide using a combined exit/entrance aperture, passing bidirectionally through a single collimating element.
As a metrology gauge, conventional interferometers require a free-space optical coupling mechanism to expand and direct an outgoing beam to a retroreflecting reference fiducial, and to efficiently receive the returning measurement beam. Older, conventionally designed interferometer instruments include large, heavy, distributed assemblies of separate optical elements, such as beam-splitters and mirrors, which are held in place by a strong metal or non-metal framework, to maintain optical alignment. Large optical elements mounted on or within large optical benches are susceptible to significant optical displacements, misalignments and distortions, due to their sensitivity to mechanical and thermal environmental inputs.
FIGS. 1 to 3 illustrate several typical problems associated with collimating lenses used by conventional interferometric metrology gauges. In FIGS. 1 and 1A, for example, light beam 100 is emitted with a numerical aperture angle NAfiber from waveguide aperture 101 in optical fiber 102, which is itself embedded within PLC 104. Commonly, in order to reduce unwanted back reflections at the fiber-air interface, the aperture face is beveled at an angle, θ1, by lapping the aperture face. In accordance with Snell's Law, light beam 100 is refracted at an angle of θ2 as it exits optical fiber 102 into free space, where it is collimated by collimating lens 105. Ideally, the collimator's principle axis is coaligned with the refraction angle θ2. If, as shown in FIG. 1A, the lens numeric aperture is smaller than the fiber numeric aperture, the collimator will not collect all the outgoing photon flux, and the interferometer will lose signal power and/or sensitivity as a result.
In FIG. 2, the optical axis of collimating lens 105 is not aligned with optical apex 202 of the corner cube retroreflector 201. This may occur due to the collimating or projection optic becoming misaligned, or the interferometer optical bench itself deforming or shifting. Light beam 100 initially emerges from optical fiber aperture 101 with numeric aperture (NAfiber) divergence 106. This beam then enters the projection optic 105 where it is collimated and projected to corner cube retroreflector 201. Due to the displacement X of the retroreflector corner cube apex 202 with respect to the collimator principle optical axis, the retroreflected return beam is laterally shifted a distance of 2X, causing a portion of returning focused beam 107 from collimator 105 to lie outside the fiber optic numeric aperture acceptance angle 106. This causes that portion of the beam to not enter the fiber optic aperture, resulting in a decrease in the intensity of the signal return, potentially compromising the required level of signal. Note that a lateral motion of the collimation optic or retroreflector perpendicular to the optical axis will not have an affect on the location of the returning focused beam, but will have an effect on the proportion of the beam return that falls within the fiber waveguide aperture acceptance angle.
In FIG. 2A, the optical axis of collimating lens 105 is not aligned with optical axis 200 of light beam 100, as it exits optical fiber 102 via aperture 101. Light beam 100 is collimated by collimating lens 105, and is reflected back towards PLC 104 by retroreflector 201. Returning, light beam 100 again passes through collimating lens 105. However, due to the lateral misalignment of lens 105, a portion of the returning beam 108 does not enter the lens and is clipped off from the collimated beam returning to waveguide aperture 101. This results in a decrease in the intensity of the signal return, potentially compromising the required level of signal.
If, as is illustrated in FIG. 3, collimator lens 105 is not optimally focused to project a collimated beam onto the corner cube retroreflector 202, then the beam returning from the retroreflector will not be perfectly focused at the entrance to the waveguide aperture 101. This will lead to the proper focus occurring before or after waveguide aperture 101, resulting in an overfill of the aperture. As with other misalignments, this leads to a reduced signal level and a lowering of the sensitivity and resolution of the metrology gauge.
One additional area of consideration which directly affects the metrology gauge performance is the wavefront quality delivered by collimator lens 105. The quality of the return beam focus, and the resulting beam fraction accepted by the waveguide aperture 101, is directly conditional on the outgoing beam quality projected by collimator lens 105, and the return beam which is retroreflected off of corner cube retroreflector 202 and returned through the same collimator lens 105, which then focuses it back onto the waveguide aperture 101. A poor quality focal beam wavefront will have significant energy in the Airy disk rings which will fall outside of the diameter of the waveguide aperture 101, thus decreasing the signal level.
As illustrated in the above examples, even small misalignments of the optical components can corrupt the signal, and have a large effect upon the ability of the conventional waveguide-type interferometer to accurately measure distances. Given that the major interferometer optical elements are discrete and are not subsumed into a simplified system, the waveguide output and return input are often ineffectively free-space coupled to the optical collimator.
Furthermore, the maintenance of an isothermal environment around the waveguide chip, especially across the opposing optical arms or channels of the waveguide, enhances the ability to obtain the maximum metrological measurement accuracy from the interferometer. Conventionally, bonding of the waveguide to a substrate is accomplished using epoxy agents which introduce a different coefficient of thermal expansion across the bond line, leading to thermally-induced stress and diminished measurement accuracy.
It is therefore considered highly desirable to provide an improved precision metrology gauge with an enhanced optical collimator and mount. In particular, it is desirable to provide for achieving a stable high measurement accuracy within a laboratory or measurement environment of diverse temperature gradients, without requiring perfect alignment of the collimator and/or retroreflecting element.